This invention relates to an atomic beam generating method and apparatus for producing a low velocity beam of atoms.
The conventional methods of producing an atomic beam are classified into the method whereby atoms placed under an elevated pressure or at a raised temperature are injected into a vacuum and the method that utilizes magneto-optical trapping as one of laser cooling techniques.
The method of injecting into vacuum requires atoms if in a solid state at a room temperature to be heated to a high temperature in an oven. An atomic beam that can be obtained by the injection method has an average velocity as high as about several hundred meters/second and a velocity distribution that largely spreads out.
In contrast, the method in which magneto-optical trapping is utilized makes it possible to produce a low velocity atomic beam that is as slow as several centimeters/second.
Magneto-optical trapping is a technique that associates a Doppler cooling process using a laser light with a central force produced according to a Zeeman shift of an atomic level by a quadruple magnetic field, which forms a low temperature atomic cloud having a temperature as low as about 1 micro-degree Kelvin. See Phys. Rev. Lett. 59, 2631 (1987), E. L. Raab et al and Applied Physics (in Japan), 60, 864 (1991), SHIMIZU, Fujio. The Doppler cooling process referred to above is a process in which atoms are irradiated with laser lights of a frequency slightly lower than the resonant frequency of the atoms, which are directed towards the atoms from six or four different directions. In this process, increases by Doppler shifts in the probabilities of absorption of the atoms moving towards the laser lights are exploited to effect three dimensional cooling thereof. See Phy. Rev. Lett. 55, 48 (1985), S. Chu et al.
As the atoms magneto-optically trapped are cooled to a temperature of 100 micro-degrees Kelvin or so, extracting the atoms trapped effectively enables a beam of atoms moving at a low velocity to be produced. In such a background, there have already been realized two methods of extraction of trapped atoms, viz. first by using a mirror with a hole designed to provide a shade for one of the cooling laser beam (see Phy. Rev. Lett. 77, 3331 (1996), Z. T. Lu et al) and second by changing the internal state of the trapped atoms (Nature, 380, 691 (1996), J. Fujita et al).
FIG. 10 is a diagrammatic view for the illustration of the conventional method in which a perforated mirror (a mirror with a hole) is used to obstruct one of cooling laser beams and to hinder it from reflecting.
In FIG. 10, there are shown vacuum chambers 70 and 71, low temperature atomic cloud 73 and 74 magneto-optically trapped and thereby held in place, laser beams 7a, 7b, 7c, 7d and 7e for three dimensionally trapping the atoms and forming such two low temperature clouds of these atoms and so holding them by Doppler-cooling the atoms. A mirror 72 is associated with one of the laser beams 7b for reflecting the laser beam 7b and is formed in its center with a hole 75. The hole 75 is designed to provide a shade for the laser beam 7b for irradiating the low temperature atomic cloud 73 therewith. The atoms located in the low temperature atomic cloud 73 thus so shaded from irradiation with the laser beam 7b gain a force directed downwards as shown in FIG. 10 and as a result a beam of the atoms is produced. The atomic beam so produced passes through the hole 75 and a transport tube 77 and is transported to the low temperature atomic cloud 74.
FIG. 10 is a diagrammatic view for the illustration of the conventional method in which a perforated mirror (a mirror with a hole) is used to obstruct one of cooling laser beams and to hinder it from reflecting.
In the method using such a hole formed mirror to provide a shade for one of cooling laser beams, however, extracting an atomic beam in an exploitable state requires the mirror to be incorporated into vacuum equipment. See Phys. Rev. A58, 3891 (1998). This poses problems such as those of the vacuum equipment becoming complicated and the mirror that may be contaminated. Furthermore, the atomic beam spreading in its velocity direction causes a portion thereof to become intercepted by the mirror, which prevents the produced atomic beam from its effective extraction.
The other method, in which the internal state of atoms is varied, irradiates the atoms with a laser light that is different in wavelength from their trapping laser light to shift the atoms to an energy level at which they do not absorb the trapping laser light, thereby releasing them from their trapped state. This renders the method applicable only to those atomic species that possess a proper energy level at which the atoms do not absorb their trapping laser light. Also, a portion of atoms that absorbed the laser light for freeing them from trapping may have shifted to an unusable energy level, which reduces the efficiency of usable extraction. See Phy. Rev. A46, R17 (1992).
It should also be noted that while atomic beams can be useful in various technical fields including high resolution spectroscopy, frequency standard, atomic wave interferometers, Bose condensation atom formation, atomic ray lithography and atomic ray surface analysis, their application to these utilizations makes it essential that they be controllable in flow rate. It has so far been difficult to control the flow rate of an atomic beam, however.
With the foregoing points taken into account, the present invention is aimed to provide an atomic beam generating method and apparatus that can produce an atomic beam with simpler vacuum equipment and at an enhanced efficiency of extraction while making its flow rate controllable and that can produce beams of atoms in an expanded range of atomic species.
In order to achieve the object mentioned above, the present invention as set forth in claim 1 in the claims appended hereto, provides an atomic beam generating method for producing an atomic beam by extracting atoms from a low temperature atomic cloud formed utilizing laser cooling, which method comprises the steps of forming a low temperature atomic cloud by irradiating the atoms with at least two sets of laser lights in a region of laser beam intersection in which they intersect, each of the sets of laser lights being made of a pair of laser beams which are opposite in direction of travel to each other, the laser beams intersecting in the said region of laser beam intersection; and providing in the said region of laser beam intersection a laser beam shading zone in which a portion of one of the laser beams in each of the sets of laser lights that is traveling in a particular direction is obstructed to provide a shade therefor, wherein the said laser beam shading zone is so located in the said region of laser beam intersection that in the said laser beam shading zone a force is brought about that is effective to force atoms in the said laser beam shading zone to move towards a predetermined direction, thereby forming a beam thereof.
In an atomic beam generating method, the present invention as set forth in claim 2 in the appended claims provides that the said laser beam shading zone is created by a tube for transporting the said beam of atoms, the said tube obstructing the said one of the laser beams in each of the sets to provide the said shade therefor.
A method as described above enables a force of high strength to push atoms to be provided and hence the atoms to be extracted efficiently, thereby producing an atomic beam effectively. Also, disusing the internal state of atoms makes the method applicable to atoms of practically all of the atomic species. Further, the method no longer requires a mirror to be incorporated in vacuum equipment and hence makes the vacuum equipment simple in construction and the mirror free from contamination.
In an atomic beam generating method, the present invention as set forth in claim 3 in the appended claims further provides adjusting the flow rate of the said beam of atoms that the said atomic beam transporting tube transports, by applying a magnetic field to the said low temperature atomic cloud to change its position in the said region of laser beam intersection so as to change the distance between the said low temperature atomic cloud and an upper end of the said atomic beam transporting tube.
Alternatively in an atomic beam generating method, the present invention as set forth in claim 4 in the appended claims further provides adjusting the flow rate of the said beam of atoms by irradiating the said low temperature atomic cloud with an additional laser beam to force atoms in the said low temperature atomic cloud aside into the said laser beam shading zone.
In an atomic beam generating method, the present invention also provides that the said additional laser beam has a wavelength with which it resonates with atoms in the said low temperature atomic cloud.
These methods make it possible to adjust the flow rate of an atomic beam being produced.
The present invention as set forth in claim 6 in the appended claims also provides an atomic beam generating apparatus for producing an atomic beam by extracting atoms from a low temperature atomic cloud formed utilizing laser cooling, which apparatus comprises: a laser system for forming a low temperature atomic cloud by irradiating the atoms with at least two sets of laser lights in a region of laser beam intersection in which they intersect, each of the sets of laser lights being made of a pair of laser beams which are opposite in direction of travel to each other, the laser beams intersecting in the said region of laser beam intersection; and a means for providing in the said region of laser beam intersection a laser beam shading zone in which a portion of one of the laser beams in each of the sets of laser lights that is traveling in a particular direction is obstructed to provide a shade therefor, wherein the said means is adapted to so locate the said laser beam shading zone in the said region of laser beam intersection that in the said laser beam shading zone a force is brought about that is effective to force atoms in the said laser beam shading zone to move towards a predetermined direction, thereby forming a beam thereof.
In an atomic beam generating apparatus, the present invention as set forth in claim 7 in the appended claims also provides that the said means for providing the laser beam shading zone comprises a tube for transporting the said beam of atoms, the said tube being arranged to obstruct the said one of the laser beams in each of the sets to provide the said shade therefor.
An apparatus as described above enables a force of high strength to push atoms to be provided and hence the atoms to be extracted efficiently, thereby producing an atomic beam effectively. Also, disusing the internal state of atoms makes the apparatus applicable to atoms of practically all of the atomic species. Further, the apparatus no longer requires a mirror to be incorporated in vacuum equipment and hence makes the vacuum simpler in construction and the mirror free from contamination.
In an atomic beam generating apparatus, the present invention as set forth in claim 8 in the appended claims further provides means for applying a magnetic field to the said low temperature atomic cloud to change its position in the said region of laser beam intersection so as to change the distance between the said low temperature atomic cloud and an upper end of the said atomic beam transporting tube, thereby adjusting the flow rate of the said beam of atoms that the said atomic beam transporting tube transports.
In an atomic beam generating apparatus, the present invention as set forth in claim 9 in the appended claims further provides that the said laser system is adapted to irradiate the said low temperature atomic cloud with an additional laser beam to force atoms in the said low temperature atomic cloud aside into the said laser beam shading zone, thereby adjusting the flow rate of the said atomic beam.
In an atomic beam generating apparatus, the present invention as set forth in claim 10 in the appended claims further provides that the said additional laser beam has a wavelength with which it resonates with atoms in the said low temperature atomic cloud.